Directions： There are 20 blanks in the following text.For each blank there are four choices marked A

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Directions： There are 20 blanks in the following text.For each blank there are four choices marked A，B，C and D.Choose the ONE that best fills the blank.

It is clear that some chemicals can damage the health of animals and humans.16　，this is not the only problem that　17　be caused by the careless use of chemicals，Chemicals can also　18　the ecological(生态的) balance of the environment.If the ecological balance is disturbed，the results can be　19　serious.

The　20　of DDT illustrates(说明) the problem.DDT，a chemical which kills insects，at first seemed to be a perfect answer　21　many problems.It would control insects that caused dangerous diseases，22　insects that caused billions of dollars of damage to crops every year.Governments permitted and even　23　the use of DDT.Farmers in many countries　24　to spray(喷洒) it on their crops.The　25　results were good：Damage to crops　26　down，and profits went up.However，the chemical had effects which the scientists didn't see in　27　.First，it also killed insects which were the　28　enemies of the harmful insects and which were therefore beneficial to farmers.Second，and perhaps worse，DDT did not kill　29　harmful insect.A few insects had natural resistance(抵抗力) to the chemical.They　30　and reproduced in large numbers.In a few years there were large numbers of insects which were not　31　by DDT，and there were　32　insects which could act as natural　33　on these new “super insects.”Finally，it became clear that DDT was not solving the insect problem.In fact，it was making the problem worse.It　34　became necessary to find a second　35　for the effects of the first.

（　　）16.A.Therefore　B.However　C.Meanwhile　D.Besides

（　　）17.A.can　B.must　C.will　D.shall

（　　）18.A.cause　B.harm　C.break　D.disturb

（　　）19.A.little　B.much　C.extremely　D.hardly

（　　）20.A.history　B.shortcoming　C.achievement　D.advantage

（　　）21.A.at　B.for　C.to　D.of

（　　）22.A.as well as　B.as well　C.but 　D.also

（　　）23.A.asked　B.requested　C.declared　D.encouraged

（　　）24.A.wanted　B.began　C.decided　D.arranged

（　　）25.A.fast　B.immediate　C.moment　D.last

（　　）26.A.put　B.took　C.broke　D.went

（　　）27.A.need　B.turn　C.advance　D.vain

（　　）28.A.natural　B.ill　C.ugly　D.friendly

（　　）29.A.any　B.some 　C.every　D.all

（　　）30.A.lived　B.died　C.cried　D.survived

（　　）31.A.effected　B.affected　C.damaged　D.killed

（　　）32.A.many　B.a few　C.a little　D.fewer

（　　）33.A.controls　B.enemies　C.choices　D.means

（　　）34.A.later　B.then　C.now　D.so

（　　）35.A.cause　B.insect　C.answer　D.cure